多相流模型经验谈

多相流模型经验谈

多相流的介绍：

Currently there are two approaches for the numerical calculation of multiphase flows: the Euler-Lagrange approach and the Euler-Euler approach.

The Euler-Lagrange Approach:The Lagrangian discrete phase model in FLUENT follows the Euler-Lagrange approach, this approach

is inappropriate for the modeling of liquid-liquid mixtures, fluidized beds, or any application where the volume fraction

of the second phase is not negligible.

The Euler-Euler Approach: In FLUENT, three different Euler-Euler multiphase models are available: the volume of fluid (VOF)

model, the mixture model, and the Eulerian model.

1)The VOF Model: it is designed for two or more immiscible fluids where the position of the interface between the fluids is

of interest. Applications of the VOF model include stratified flows, free-surface flows, filling, sloshing, the motion of

large bubbles in a liquid, the motion of liquid after a dam break, the prediction of jet breakup (surface tension), and the

steady or transient tracking of any liquid-gas interface.

2) Mixture model:Applications of the mixture model include particle-laden flows with low loading, bubbly flows, sedimentation,

and cyclone separators. The mixture model can also be used without relative velocities for the dispersed phases to model homogeneous multiphase flow.

3)Applications of the Eulerian multiphase model include bubble columns, risers, particle suspension, and fluidized beds.

离散相模型（离散相的装载率10～12%）

求解参数的设定：

Options for Interaction with Continuous Phase：For steady-state simulations, increasing the Number of Continuous Phase

Iterations per DPM Iteration will increase stability but require more iterations to converge.

Update DPM Sources Every Flow Iteration is recommended when doing unsteady simulations; at every DPM Iteration, the particle

source terms are recalculated.

Length Scale: controls the integration time step size used to integrate the equations of motion for the particle.A smaller

value for the Length Scale increases the accuracy of the trajectory and heat/mass transfer calculations for the discrete phase.

Length Scale factor: A larger value for the Step Length Factor decreases the discrete phase integration time step.

颗粒积分方法：numerics叶中tracking scheme选项

1）implicit uses an implicit Euler integration of Equation 23.2-1 which is unconditionally stable for all particle relaxation times.

2）trapezoidal uses a semi-implicit trapezoidal integration.（梯形积分）

3）analytic uses an analytical integration of Equation 23.2-1 where the forces are held constant during the integration.

4）runge-kutta facilitates a 5th order Runge Kutta scheme derived by Cash and Karp [47].

You can either choose a single tracking scheme, or switch between higher order and lower order tracking schemes using an

automated selection based on the accuracy to be achieved and the stability range of each scheme.

Max. Refinements is the maximum number of step size refinements in one single integration step. If this number is exceeded

the integration will be conducted with the last refined integration step size.

Automated Tracking Scheme Selection provides a mechanism to switch in an automated fashion between numerically stable lower

order schemes and higher order schemes, which are stable only in a limited range. In situations where the particle is far

from hydrodynamic equilibrium, an accurate solution can be achieved very quickly with a higher order scheme, since these

schemes need less step refinements for a certain tolerance. When the particle reaches hydrodynamic equilibrium, the higher

order schemes become inefficient since their step length is limited to a stable range. In this case, the mechanism switches

to a stable lower order scheme and facilitates larger integration steps.

Including a Coupled Heat-Mass Solution on the Particles：This increased accuracy, however, comes at the expense of increased computational expense.

非稳态跟踪

1）连续相稳态离散相非稳态：you simply enter the Particle Time Step Size and the Number of Time Steps, thus tracking particles

every time a DPM iteration is conducted. When you increase the Number of Time Steps, the droplets penetrate the domain faster.

2）连续离散相都为非稳态：When solving unsteady equations for the continuous phase, you must decide whether you want to Use

Fluid Flow Time Step to inject the particles, or whether you prefer a Particle Time Step Size independent of the fluid flow

time step. With the latter option, you can use the Discrete Phase Model in combination with changes in the time step for

the continuous equations, as it is done when using adaptive flow time stepping.

随机轨道模型的参数：

number of tries:An input of zero tells FLUENT to compute the particle trajectory based on the mean continuous phase velocity

field (Equation 23.2-1), ignoring the effects of turbulence on the particle trajectories. An input of 1 or greater tells

FLUENT to include turbulent velocity fluctuations in the particle force balance as in Equation 23.2-20.

If you want the characteristic lifetime of the eddy to be random (Equation 23.2-32), enable the Random Eddy Lifetime option.

You will generally not need to change the Time Scale Constant (CL in Equation 23.2-23) from its default value of 0.15,

unless you are using the Reynolds Stress turbulence model (RSM), in which case a value of 0.3 is recommended.

液滴颗粒碰撞与破碎

碰撞：

破碎：有两种模型，TAB 模型适合低韦伯数射流雾化以及低速射流进入标态空气中的情况。对韦伯数大于100 的情况，波动模型适应性较好。 在高速燃料射流雾化中，波动模型应用甚广。

对于TAB 模型，用户需要在y0 文本框中设定y0的值。The default value (y0 = 0) is recommended.

对于Y波动模型，需要输入B0与B1，you will generally not need to modify the value of B0, as the default value 0.61 is acceptable

for nearly all cases. A value of 1.73 is recommended for B1.

颗粒类型中的燃烧类型

燃烧（``combusting''）颗粒是一种固体颗粒，它遵从由方程19.2-1 所确定的受力平衡、由定律1 所确定的加热冷却过程、由定律4 所确定

的挥发份析出过程（19.3.5 节）以及由定律5 所确定的异相表面反应机制（19.3.6 节）。最后，当颗粒的挥发份完全析出之后，非挥发份 的运动、变化由定律6 所确定。在Set Injection Properties panel 面板中选定Wet Combustion 选项，用户可以在燃烧颗粒中包含有可蒸发

物质。这样，颗粒的可蒸发物质可在挥发份开始析出之前，经历由定律2、3 所确定的蒸发与沸腾过程.

若定义的是Combusting 燃烧类型颗粒，可在Devolatilizing Species 下拉列表框下选定由挥发份析出定律4 确定的气相组分，参与焦炭表面

燃烧反应（定律5）的气相组分列于Oxidizing Species（氧化剂组分）列表中，有表面反应生成的气相组分则列于ProductSpecies（生成物组

分）列表中。需要注意的是，对于选定的燃烧颗粒介质，如果燃烧模型为multiple-surface -reaction 多表面异相反应模型，那么，由于化学 反应计量比在混合介质中已经被确定，所以Oxidizing Species 与Product Species 列表将变灰（不可选）。

液滴喷射类型

平面雾化模型的输入

l 位置：在X-, Y-, and Z-Position 文本框区可以设定射流的沿直角坐标的三向位置（在三维情况下才会有Z-Position 出现）

l 速度：在X-, Y-, and Z- Velocity 文本框区可以设定射流初始速度沿直角坐标的三向分量 （在三维情况下才会有Z- Velocity 出现） l 轴的方向（仅适用于三维）：设定确定喷嘴轴线方向的三个分量，在X-Axis, Y-Axis, and Z-Axis 区设定。

l 温度：在Temperature 区可设定喷射颗粒流的初始颗粒（绝对）温度。 l 质量流率：可在Flow Rate 区设定喷嘴的的颗粒质量流量。 l 射流持续时间：对于非稳态颗粒跟踪计算（请参阅19.8 节），在Start Time 和Stop Time 区设定喷射的开始于结束时间。

l 蒸气压：设定控制通过喷嘴内部流动的蒸气压（表19.4.1 中的pv ），在Vapor Pressure 区设定。

l 直径：设定喷嘴直径（表19.4.1 中的d ），在Injector Inner Diam.区设定。 l 喷嘴长度：设定喷嘴的长度（表19.4.1 中的L ），在Orifice Length 区设定。 l 内台阶角半径（导角半径）：设定喷嘴内台阶处的导角半径（表19.4.1 中的r ），在Corner Radius of Curv.区设定。

l 喷嘴参数：设定射流角修正系数（方程19.4-16 中的 CA ），在Constant A 区设定。{CA=3+L/3.6/d,喷射角度的大小强烈依赖于喷嘴的

内部流动。因此，对于空穴喷嘴，用户设定的CA 值应该比单相流的要小才可以。CA 的常见取值范围为4.0～6.0。返流喷嘴的喷射角度更小 }

l 方位角：设定三维情况下的喷嘴方位开始角与结束角，在Azimuthal Start Angle and

Azimuthal Stop Angle 区设定。

压力－旋流雾化喷嘴的点属性设定(气体透平工业的人把它称作单相喷嘴（simplex atomizer）。这种喷嘴，然后流体通过一个称作旋流片

的喷头被加速后，进入中心旋流室。在旋流室内，旋转的液体被挤压到固壁，在流体中心形成空气柱，然后，液体以不稳定的薄膜状态从 喷口喷出，破碎成丝状物及液滴。)

l 射流角：在Spray Half Angle 区下设定射流喷射半角（方程19.4-25 中的θ ）。 l 压力：在Upstream Pressure 区下设定喷嘴上游压力（表19.4.1 中的p1 ）。

l 液膜破碎常数：设定确定液膜破碎时形成的线状液膜长度的一个经验常数（方程19.4-30中的ln（ηb/η0），在Sheet Constant 设定。 {ln（ηb/η0）为3～12 的经验常数。这个值必须由用户设定，其缺省值为12 with experimental sheet breakup lengths over a range of Weber numbers from 2 to 200.}

l 线状液膜直径：对于短波，确定液膜破碎波长与线状液膜半径之间的线形比例关系的比例常数，在Ligament Constant 区设定。

{where CL, or the ligament constant, is equal to 0.5 by default.}

空气辅助雾化喷嘴的点属性设定(为了加速液膜的破碎，喷嘴经常会添加上辅助空气。液体通过喷座的作用形成液膜,空气则直接冲击液膜 以加速液膜的破碎。)

l 喷嘴外半径：在Injector Outer Diam. 区下设定射流的外部半径。此数值与喷嘴内部半径共同确定了液膜厚度（方程19.4-22 中的t ）。 l 射流角：设定射流离开喷口时的液膜初始轨道（方程19.4-25 中的θ ），在Spray Half Angle 区设定。

l 相对速度：设定液膜与空气之间的最大相对速度，在Relative Velocity 区设定。

l 液膜破碎常数：设定确定液膜破碎时形成的线状液膜长度的一个经验常数（方程19.4-30中的ln（ηb/η0）），在Sheet Constant 区设定。

l 线状液膜直径：对于短波，确定液膜破碎波长与线状液膜半径之间的线形比例关系的比例常数，在Ligament Constant 区设定。

{where CL, or the ligament constant, is equal to 0.5 by default.}

平板扇叶雾化喷嘴的点属性设定(液体从宽而薄的喷口出来后形成平面液膜，继而破碎成液滴。只有在三维的情况下才可以使用这个模型)

l 扇叶中心点：设定射流源起始位置的三向坐标值（请参阅图19.4.6），在X-Center,Y-Center, and Z-Center 区设定。

l 虚点位置：设定喷嘴扇叶的各边的虚拟交叉点（请参阅图19.4.6），在X-Virtual Origin, Y-Virtual Origin, and Z-Virtual Origin 区设定。

l 垂直方向：设定垂直扇叶的向量的各个分量，在X-Fan Normal Vector, Y-Fan Normal Vector, and Z-Fan Normal Vector 区设定。

l 温度：设定颗粒流的温度，在Temperature 区设定。

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